some thoughts and comments about the railway track

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Evaluation of the track quality

Track measurement

The importance of knowing the track geometric quality arose in the middle of 20th century, when European infrastructure managers developed their own track recording vehicles allowing a continuous measurement of track geometry and based on this, their own track geometry quality evaluation standards (EN 13848-6).

Periodic track measurement is required to evaluate the track quality and maintain an effective railway track system – safe and with good vehicle ride quality.

The track quality output requires as a minimum the following (EN 13848-6):

Isolated defects that exceed a prescribed threshold and require intervention. This defect review is ensuring the safety of the railway track system.

A Track Quality Index (TQI). This is produced over a specific length of track and is the one that defines the riding quality.

Track defect

Any track parameter is defined by a nominal value which is the characteristic of an ideal track.

For example, the gauge nominal value is 1435 mm. Some railway networks define for the normal gauge and some specific applications variations of this figure: 1432, 1433, 1438. All these are nominal gauge values for those track applications.

The installation, usage and maintenance of the railway track do not allow installing and maintaining the track parameters exactly to their nominal value. A real track will be always characterised by variations of the nominal values for all the track parameters.

In the case of gauge we will find the nominal value of 1435 mm only by chance. The measured gauge will be in a (narrow or wide) range around its nominal value.

All the variances from the nominal value define the track irregularities.

In order to evaluate the track quality, these track irregularities are divided into two main categories:

tolerance – accepted variance from the nominal value

defect (fault) – nominal value variance that is not acceptable and require intervention to keep the track system safe.

The railway administrators define, for each of the relevant track parameters and dependent on speed or other factors, three main levels of action:

Alert Limit (AL) – the irregularity is still tolerated but requires attention.

Intervention Limit (IL) – the track irregularity has become a defect and correction needs to take place within a certain period of time.

Track Quality Index (TQI)

Some of the Track Quality Indexes used by the European Railway Networks are (EN 13848-6):

The number of insulated defects over a specific length(e.g. 1km). This is a very simple way of evaluating the track quality and is still used by several European Railway Networks. Each defect and, sometimes, tolerated irregularity close to the defect limit, is scored and summed to produce the general defect scoring of that length of track.

Parameters defined through the Vehicle Response Analysis (VRA). This is either a directly measured parameter or the product of a mathematical model that quantifies the vehicle response to the track irregularities.

Standard Deviation (SD). It is calculated for each specific wavelength range over a defined length, typically 200m. There is a direct relationship between Standard Deviation (SD) and the Power Spectral Density (PSD).

Combination of various parameters. For example the TQI can be expressed as a combined standard deviation (CoSD) computed based on the different set of SD considered in the measurement of track quality (for alignment, cant, gauge, cross level, longitudinal level).